The present invention relates to corona generating devices and more particularly to such a device which is utilized for partially neutralizing electrostatic charges on a surface utilized in the xerographic process.
Generally, the process of xerographic or electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, it is developed by bringing a developer mixture into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. Finally, the powder image is heated to permanently affix it to the copy sheet in image configuration.
Corona generating devices are commonly used for charging the photoconductive member and for effecting transfer of the toner images to the copy sheet. During the transfer process the backside of the copy sheets is charged to a suitable level and polarity to effect transfer of the images to the copy sheet. By virtue of the charge on the backside of the copy sheet a charge of opposite polarity is induced on the photoconductive member in the non-image areas thereby creating an electrostatic bond between the copy sheet and the photoconductive member.
To facilitate separation of the copy sheet from the photoconductive member, it is desirable to partially neutralize the aforementioned charges on the backside of the copy sheet to thereby remove a part of the electrostatic bond caused thereby. Corona devices have also been employed for this purpose. The process by which this is accomplished has come to be known as electrostatic detacking. Another application is that of "preclean" where the toner charge and/or photoreceptor charge may need to be neutralized.
The conventional form of corona discharge device used in reproduction systems of the above type whether for charging, transfer or detack is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating d.c. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an a.c. corona generating potential is applied to the conductive wire electrode and a d.c. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged.
A problem associated with conventional corona discharge devices employing a conductive wire is a result of the fact that corona glow is associated with a region of high chemical reactivity where chemical compounds are synthesized from machine air, which results in chemical growths being built up on the surface of the wire. These chemical growths, after a prolonged period of operation, degrade the performance of the corona device. Since free oxygen and ozone are produced in the corona region the corona electrode must of necessity be highly oxidation resistant. The above problem of chemical growth build-up on the wire has been addressed by the provision of wire materials which are less subject to chemical attack. While this has reduced the problem, such materials have substantially increased the cost of corona devices.
Prior art devices utilizing a bare wire when operated in an a.c. mode with the shield grounded, do not completely neutralize a charged surface. The prior art corona device delivers a net negative d.c. current (-Ip) to the collecting surface when that surface is completely neutralized (Vp=0). Another problem which "bare" wire devices is that of lengthwise current non-uniformity.
The above characteristic of prior art a.c. corona devices results from the greater mobility of negative ions. This phenomenon is well known in the art. The use of d.c. power supplies for biasing the shield of such corona devices has been employed to make them operational in a desired manner. While such external biasing arrangements allow for the desired operation of an a.c. corona device with the output of corona generators operated in this manner there is a high probability of change due to ambient conditions such as temperature and humidity and contamination conditions due to toner accumulation.
Another type of corona generating device considered more suitable for the xerographic charging functions is the dicorotron electrode which is the conbination of a conductive wire and a relatively thick dielectric outer coating as disclosed in U.S. Pat. No. 4,086,650. The dicorotron is not as susceptible to the aforementioned shortcomings as is the bare wire corotron. The thickness of the dielectric is such that with an a.c. voltage applied to the dicorotron electrode and with the shield and surface to be exposed to corona at the same potential there is substantially no net d.c. current flow between the wire and the surface nor between the wire and the conductive shield forming a part of the corona generating device. However, if an electrostatic field is created between the shield and the surface as by applying a bias to the shield or placing a charge on the surface then there would be a net d.c. current flow to the surface and to the shield.
Another feature of the dicorotron is that unlike the bare wire corotron the current flowing between the dicorotron wire and the surface to be exposed to corona is equal in magntitude and opposite in polarity to the current flowing between the dicorotron wire and the conductive shield.
In the case of the bare wire corotron with an a.c. voltage applied, the two currents are of the same polarity and usually of different magnitudes since it is desirable to have more current flowing to the surface being exposed to corona. To obtain greater current flow to the photoconductive surface when used for charging a current limiting resistor has been used as disclosed in U.S. Pat. No. 3,813,548 issued in the name of Morton Silverberg.
The dicorotron, therefore, is particularly suited for partially neutralizing the charge place on the backside of the copy sheet during transfer. This is because the voltage on the backside of the copy sheet when it and the photoconductive surface are moved past the detack dicorotron creates an electrostatic field between the shield and the copy sheet which causes a net d.c. current to flow to the backside of the copy sheet thereby neutralizing part of the charge thereon.
As may be appreciated, the voltage level on the backside of the copy sheet will not be a constant value due to the variety of operating conditions to which a xerographic machine is subjected. Such machines must be capable of handling various weights of paper under various atmospheric conditions. The thickness of the paper which is a function of its weight and the paper's resistivity which is a function of atmospheric conditions and the type of paper are factors which alter the voltage level that appears on the backside of the copy sheet after transfer. Such variations in copy sheet voltage levels have been handled in prior art devices by the provision of costly and complex feedback circuits for sensing the voltage levels on the backside of the copy sheet and using the sensed value to derive a signal which is used to charge the power output of the shield bias power supply.